The "easy way" MTH DE0000018 universal diesel tire. https://www.mthpartsandsales.com
The "hard way" Go searching on Lionel Support using the product number https://www.lionelsupport.com/...-Passenger-Train-Set
Then click on the powered engine https://www.lionelsupport.com/...d-A-Unit-600-6-14579
Then scroll dow the list. Notice the alternate number "6208613206"
So I search using that number https://www.lionelsupport.com/...?keywords=6208613206
At least 2 different ones are "in stock".